1. Field of the Invention
The present invention relates to a method and apparatus for aquiring and superimposing two acoustic images, one generated from the energy of light radiated into a subject being examined and the second is an ultrasound echo image generated from ultrasonic waves directed into the subject being examined superposition of the two images yields a distribution of substance concentration with respect to morphological features in the subject's tissue.
2. Description of the Related Art
A method of acquiring information about a subject has been used to measure the concentration of a component contained in a body fluid such as blood, other body fluids in the subject or excised tissue, thereby to achieve accurate diagnosis, fast determination of treatment course, and improved healthcare. To measure the concentration of each component of a body fluid, the body fluid must be extracted from the subject by blood collection. The extraction of the fluid is painful. It damages the skin of the subject and introduces the possibility of biohazard contamination to the subject and the operator.
To solve this problem, a number of patents and journal articles describe non-invasive methods of acquiring information about analyte concentration in the tissue of human subjects. One of the methods is “photoacoustic spectroscopy”. In the photoacoustic spectroscopy, the concentration of a specific substance, such as glucose or hemoglobin, contained in the blood of a subject is quantitatively determined from the acoustic waves that are generated when the subject's tissue is irradiated with short duration pulses of light of a predetermined wavelength. U.S. Pat. No. 5,348,002, WO9838904A1, WO0215776A1 describe methods for the non-invasive determination of substances in human tissue using photoacoustic measurements. The light may be visible light, infrared light, or intermediate infrared light.
In addition to hemoglobin and glucose, photoacoustic spectroscopy can be used to determine other analytes in human tissue such as cholesterol, natural fat, bilirubin, collagen, and the like. Diagnosis of cutaneous cancer or breast cancer based on the results of the photoacoustic spectroscopy has recently proven its clinical usefulness. The photoacoustic spectroscopy utilizes a suitable substance selected from these substances and light having a wavelength at the substance selected exhibits highest absorption. Further it is increasingly expected that a diagnosis method be invented, which provides a two-dimensional image representing the concentration distribution of these substance.
While photoacoustic spectroscopy is used to measure substance concentration in tissue, ultrasound imaging has been extensively used for determination of the presence of morphological features, such as cysts and lumps, in human organs. Combining the distribution of substances and the morphological features in human tissue leads to better diagnosis and improved healthcare as it provides better characterization of the tissue, more accurate diagnosis for malignancies, and better definition of regions of abnormal pathology to guide in surgical removal of these regions.
Breast cancer is a major source of mortality in females. Screening for and early diagnosis of breast cancer is of tremendous value in cutting mortality rate and in health care cost containment. Current methods involve manual examination of breast tissue for unusual lumps and routine mammography to look for suspicious lesions. In a mammogram is deemed suspicious, it is followed by ultrasound imaging, and surgical biopsy. These set of steps take considerable time before reaching a final conclusion.
Non-invasive optical techniques offer the opportunity for determining blood vessel distribution in tissue, thus locating a potential tumor by the presence of abnormal vascularization in a tissue region. Non-invasive optical techniques include time resolved light propagation in tissue. Another method is the measurement of the change in modulation and phase angle as photon density wave propagate in the tissue. These are presented is several journal articles (B. Chance “Near-infrared images using continuous, phase-modulated, and pulsed light with quantitation of blood and blood oxygenation’ in Advances in Optical Biopsy and Optical Mammography, R. Alfano ed, Annals of the New York Academy of Sciences 1998; Volume 838: pages 29–45; by S. Fantini et al “Frequency domain optical mammography: Edge effect corrections” Medical Physics 1996; Volume 23: pages 1–6, and by M. A. Franceschini et al “Frequency Domain techniques enhance optical mammography; initial clinical results” Proceedings of the National Academy of Sciences USA 1997; Volume 94: pages 6468–6473 (1997)). These methods suffer from imprecision of image conversion and image distortions close to the edges of the body part, such as the breast.
Imaging methods of the art that includes ultrasound, CAT scan, X-ray and MRI describe the morphology of the body part, in this case the breast without indicating the distribution of hemoglobin. Further, MRI and CAT scan have large expensive equipment that cannot be transformed easily.
A diagnostic method and apparatus that utilizes the morphological image and the distribution of subatances in the morphological feature leads to better diagnosis. Use of photoacoustic imaging to determine analyte distribution in breast tissue was described by A. A. Oraevsky et al “Laser opto-acoustic imaging of breast: Detection of cancer angiogenesis” SPIE Proceedings 1999; Volume 3597, pages: 352–363; and A. A. Oraevsky et al “Opto-acoustic imaging of blood for visualization and diagnostics of breast cancer” SPIE Proceedings 2002; Volume 4618, pages: 81–94. It is also described in the patent art in U.S. Pat. No. 5,840,023 “Optoacoustic imaging for medical diagnosis”, WO 01/10295 “Photoacoustic monitoring of blood oxygenation”, and U.S. Pat. No. 6,309,352 B1 “Real Time optoacoustic monitoring of changes in tissue properties”.
Oraevsky et al use photoacoustic imaging alone without combination with ultrasound imaging. They do not teach combination of photoacoustic and ultrasound images that are detected using co-registered ultrasound transducers. The method leads to the possibility of distortion of the vascular image due to effect of the morphological features on tissue bulk modulus.
Other application of optical methods to generate an image of analyte distribution in tissue is described by Q. Zhu et al in “Combined ultrasound and optical tomography imaging” SPIE Proceedings 1999; Volume 3579, pages: 364–370; and Q. Zhu et al “Optical imaging as an adjunct to ultrasound in differentiating benign from malignant lesions” SPIE Proceedings 1999; Volume 3579: pages 532–539. Zhu et al uses ultrasound imaging to define the morphological features in tissue and then apply frequency domain imaging to determine vascularization i.e. hemoglobin distribution. Optical fibers and photomultiplier tubes are used as detectors for the optical method and ultrasound transducers are used for ultrasound imaging with less optimum co-registration between the vascularization and the morphological images. Does not teach combination of photoacoustic and ultrasound images that are detected using co-registered ultrasound transducers.
In a conventional non-invasive method of measuring glucose, the skin of the subject is irradiated with near-infrared light beams of different wavelengths. The glucose concentration is measured by arithmetically processing the acoustic waves obtained (see Jpn. Pat. Appln. KOKAI Publications Nos. 3-47099 and 5-58735).
The conventional photoacoustic spectroscopy uses a microphone and a piezoelectric element made of lead zirconate titanate (PZT) ceramics, or the like, to detect acoustic waves (see Jpn. Pat. Appln. KOKAI Publications Nos. 10-189 and 11-235331).
Research has been conducted on imaging methods using the photoacoustic effect for diagnosing breast cancer (see Jpn. Pat. Appln. KOKAI Publications Nos. 3-47099). FIG. 13 illustrates the system 100 for acquiring photoacoustic image data, described in non-patent reference 1. The system 100 comprises a laser generator 101, an optical fiber 103, an array of electroacoustic transducer elements 104, and a computer system 105. The laser generator 101 generates light pulses. The optical fiber 103 guides the light pulse to the breast 102 of a subject to be examined. The electroacoustic transducer elements 104 are placed, facing the optical fiber 103. Each element 104 has a concaved surface. The computer system 105 controls transmission of optical pulses, acquires acoustic waves, and reconstructs an image.
The subject lies on a table, with the breast 102 positioned between the optical fiber 103 and the array of electroacoustic transducer elements 104. Then, the tissues in the breast 102 are irradiated with light (laser beam) applied from the optical fiber 103. The blood components in the internal tissues generate acoustic waves. The electroacoustic transducer elements 104 receive the acoustic waves.
In this method, the concentration of hemoglobin in blood, for example, can be measured with higher sensitivity than the concentration of any other substance components, by virtue of the photoacoustic effect based on a predetermined wavelength. Therefore, a photoacoustic image of a tumor tissue in the breast can be more readily detected than an image obtained by an ultrasonic diagnosis apparatus, X-ray apparatus, MRI apparatus, or the like, which has hitherto been used. This is because vascularization, which is the number of blood vessels, and the blood flow rate are higher in the tumor tissue than in normal tissues, in order to accommodate the higher metabolic activity in the tumor. Increased vascularization occurs through generation of more blood vessels in the tumor and its surroundings. Generation of new blood vessels in tumors is known as angiogenesesis.
The methods disclosed in Jpn. Pat. Appln. KOKOKU Publication Nos. 3-47099 and 5-58735, and Jpn. Pat. Appln. KOKAI Publication Nos. 10-189 and 11-235331 are designed to measure the concentration of a specific substance in a local region. However, none of these publications teaches techniques of providing an image showing concentration distributions.
The method described in A. A. Oraevsky, et al., “Laser optoacoustic imaging of breast cancer in vivo”, Proc. SPIE, Vol. 4256: pages. 6–15, 2001, lacks operability. This is because, the optical fiber 103 and an array of electroacoustic transducer elements 104 opposite to each other, and the subject lies with the breast 102 held between them. It is desirable to form the optical fiber 103 integral with the array of electroacoustic transducer elements 104, because air must be expelled, as much as possible, from the gap between the array and the subject, particularly when an image is reconstructed from the acoustic waves generating from inside the subject.
In addition, image reconstruction using such acoustic waves (referred to as “photoacoustic imaging method” hereinafter) is performed only for a particular component such as hemoglobin. Hence, no signals can be obtained from any region than contain without the component. Therefore, when the photoacoustic imaging method is performed to examine the breast for cancer as described in A. Oraevsky et al., “Laser optoacoustic imaging of breast cancer”, it is difficult to determine an accurate positional relationship between a tumor and morphological features in the tissue and such as healthy mammary gland tissue surrounding it.
Thus there is a need to develop methods and apparatus for diagnosing disease states by combining imaging of morphological features and distribution of substance concentration within the features, while avoiding image distortion, incorporating common body interface and common detectors, for the imaging measurement and the substance distribution measurement. The method and the apparatus should lead to applying the same pressure, same air gaps, same interfaces for the imaging measurement and the substance distribution measurement. The purpose of this invention is to fulfill this need.